Human is the most well-rounded race. Pretty much any character class can make excellent use of attack bonus, and the benefits of attack bonus scale very well as you level up. Although the human is overshadowed by the Orc at lower levels, he shines at the higher levels.

Human vs Orc Comparison

The human's conversion provides attack bonus, while the orc's conversion bonus provides stacking base damage. Understanding the difference between these two is important to knowing when to use humans and when to use Orcs.

Damage is calculated by multiplying base attack by attack bonus. Attack bonus improves as you find attack boosters on the ground, while base attack improves every time you level up. This means orcs tend to perform very well at low levels when your base attack is low and their conversion benefit is a relatively big deal. Humans will slowly catch up to orcs as you level up, ultimately being stronger at higher levels when you're preparing for the boss fight.

In general, it's ideal to balance some bonuses to base attack and attack bonus. This generally produces the best overall attack damage. There are more class benefits, power-ups, and items that improve your attack bonus than there are those that improve your base attack, which can make the orc more appealing in most dungeons. However, if your class does not have an increase to attack bonus (such as the Paladin) then the human is more favourable.

Strategy

The human conversion bonus isn't very significant at lower levels, so it's often worthwhile to keep utility glyphs in your inventory. However, because their racial bonus is passive you can really benefit over the long-term from repeated conversions. At higher-levels, the human matures into easily the best race for a brawling combatant, and damage above 100 per hit at level 10 is not uncommon.

Because humans deal more damage per attack, they can often come out ahead of monsters when it comes to healing, allowing them to whittle down stronger foes just by repeatedly exploring to heal and then returning to fight. This also makes them exceptional poison users. Damage resistance is a great way to further leverage the benefits of a human, and glyphs that allow for an extra free attack are well worth your mana to use.

Humans perform well for pretty much any class. The only classes they are not as good for are the Crusader and the Rogue, which prefer the Orc due to the fact that their attack bonus is already very high from their class features.